1. Field of the Invention
The present invention relates to a driver circuit of a display device, and in particular, to the driver circuit of the display device of a self-luminous type such as an organic EL (Electro Luminescence) of which output precision is required.
2. Description of the Prior Art
It is a known fact in recent years that an information electronics device such as a portable telephone is widely used in the world. It is also well known that the information electronics device has a self-luminous type display device such as an organic EL as its display apparatus. A matrix type display device is also well known as one of the representatives of the self-luminous type display devices such as the organic EL.
The display device shown in FIG. 1 or FIG. 2 is also known as such a matrix type display device, for instance.
The matrix type display device in the past 2100 shown in FIG. 1 has a configuration wherein a plurality of data lines (not shown) connected to a data line driver circuit 2103 and a plurality of scanning lines connected to a scanning line side driver circuit 2102 are provided, and a display panel 2101 having a liquid crystal, the organic EL and so on is provided at each intersection thereof.
The matrix type display device in the past 2200 shown in FIG. 2 has a configuration wherein a plurality of data lines (not shown) connected to a data line driver circuit 2203 and a plurality of scanning lines connected to a scanning line side driver circuit 2202 are provided, and a display panel 2201 having the liquid crystal, organic EL and soon is provided at each intersection thereof.
FIG. 3 is an equivalent circuit diagram of a TFT (Thin Film Transistor) liquid crystal cell 1701 using a TFT 1703 as an active element, wherein transmittance is controlled by voltage. FIG. 4 is an equivalent circuit diagram of an organic EL cell 1801 using two TFTs (1803, 1806), wherein luminance is controlled by voltage. FIG. 5 is an equivalent circuit diagram of a simple matrix type organic EL cell 1901, and FIG. 6 is an equivalent circuit diagram of an organic EL cell 2001 using four TFTs (2003, 2006, 2008, 2009), wherein luminance is controlled by currents.
A voltage-control type data driver circuit 1400 of the matrix type display device in the past selects on a gradation voltage selection circuit 2 one voltage value, according to digital image data, of a plurality of voltages generated on a gradation voltage generation circuit 1 (refer to FIG. 7) so as to drive the data lines via an amplifier 4.
When the number of bits of the digital image data increases, the gradation voltage selection circuit 2 increases impedance in order to reduce the area of constituting elements because a chip-occupied space thereof becomes larger in proportion to the number of bits. For that reason, the data lines are driven by having the voltage selected on the gradation voltage selection circuit 2 impedance-converted by an amplifier 4.
In general, a liquid crystal display has a driving voltage range of 3 to 5V, and the digital image data is 4 to 6 bits in the case of the portable telephone and so on.
And a current-control type data driver circuit drives the data lines with a plurality of weighted current sources 31 as shown in FIG. 8.
The data driver circuit of the display device is generally integrated, and has the same number of output terminals as the number of horizontal data lines of the display device. Or in the case where a plurality of data lines are connected to one driver circuit in parallel as shown in FIG. 2, the data driver circuit of the display device has the number of output terminals which is the number of pixels/the number of parallels added thereto, so that the number of the output terminals thereof becomes a few tens to a few thousands or more. As for semiconductor equipment and so on, manufacturing variations cause voltage variations and current variations.
For that reason, Japanese Patent Laid-Open No. 4-142591 proposes a method of, in order to reduce output voltage variations of the data driver circuits of the liquid crystal display device, having the data of which output voltage variations are to be corrected stored by storage circuit in advance and driving the liquid crystal with a signal wherein the data of the storage circuit in synchronization with a clock signal is added to a picture signal.
However, the following problem arises as to the method of adding the digital image data and correction data as with the data driver circuit of the liquid crystal display described in Japanese Patent Laid-Open No. 4-142591.
In the case of the liquid crystal display, a voltage difference capable of recognizing display variations of the liquid crystal is approximately 5 mV or so. In the case where the driving voltage range of the liquid crystal is 3V, it is 3000 mV/5 mV=600 and precision of 9 bits (512-value) or higher is required. To be more specific, 9-bit or more correction data is necessary to correct the voltage variations of the driver circuits.
Even in the case where the digital image data is 6 bits, the circuits from an addition circuit onward are 9 bits or more, and so the circuit scale of the data driver circuit becomes larger.
In addition, a voltage-to-transmittance property of the liquid crystal (FIG. 9) and a voltage-to-luminance property of the organic EL (FIG. 10) are nonlinear and so a correction amount is different according to the voltage. Therefore, as the digital image data cannot be simply added to the correction data, the correction data for each piece of the digital image data is required and thus a correction data storage circuit becomes even larger.
An organic EL display device has a luminance-to-current property which is linear, and so it is driven by the plurality of weighted current sources. In this case, as can be easily presumed from Japanese Patent Laid-Open No. 4-142591, there is a thinkable method of correcting the current value by storing the data for correcting output current variations in advance. However, there are the cases where, as each of the weighted current sources varies independently, a monotone increase property is lost, and the correction data storage circuit becomes enormous because each bit of the digital image data requires the correction data.
Furthermore, the variations at the time of manufacturing are stored in an ROM or the like in order to store the variations in the driver circuits as the correction data in advance, so that the variations cannot be corrected against the change in use conditions (change in temperature and change over time)